This document gives instructions for building Perl for RISC OS.
It is complicated by the need to cross compile.
There is a binary version of perl available from http://www.cp15.org/perl/ which you may wish to use instead of trying to compile it yourself.

Firstly,
copy the source and build a native copy of perl for your host system.
Then,
in the source to be cross compiled:

$ ./Configure

Select the riscos hint file. The default answers for the rest of the questions are usually sufficient.

Note that, if you wish to run Configure non-interactively (see the INSTALL document for details), to have it select the correct hint file, you'll need to provide the argument -Dhintfile=riscos on the Configure command-line.

$ make miniperl

This should build miniperl and then fail when it tries to run it.

Copy the miniperl executable from the native build done earlier to replace the cross compiled miniperl.